docs: separate template syntax into multiple docs (#36954)

This is part of a re-factor of template syntax and
structure. The first phase breaks out template syntax
into multiple documents. The second phase will be
a rewrite of each doc.

Specifically, this PR does the following:

- Breaks sections of the current template syntax document each into their own page.
- Corrects the links to and from these new pages.
- Adds template syntax subsection to the left side NAV which contains all the new pages.
- Adds the new files to pullapprove.

PR Close #36954

packages/common/src/directives/ng_for_of.ts

@@ -186,7 +186,7 @@ export class NgForOf<T, U extends NgIterable<T> = NgIterable<T>> implements DoCh
 
   /**
    * A reference to the template that is stamped out for each item in the iterable.
-   * @see [template reference variable](guide/template-syntax#template-reference-variables--var-)
+   * @see [template reference variable](guide/template-reference-variables)
    */
   @Input()
   set ngForTemplate(value: TemplateRef<NgForOfContext<T, U>>) {

packages/common/src/directives/ng_if.ts

@@ -100,7 +100,7 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef, ɵstri
  *
  * The conditional displays the data only if `userStream` returns a value,
  * so you don't need to use the
- * [safe-navigation-operator](guide/template-syntax#safe-navigation-operator) (`?.`)
+ * [safe-navigation-operator](guide/template-expression-operators#safe-navigation-operator) (`?.`)
  * to guard against null values when accessing properties.
  * You can display an alternative template while waiting for the data.
  *